In electric generators, energy is transferred between a mechanical state and an electrical state by means of a generated magnetic field and windings in a rotating rotor and a stationary stator, in a well-known manner. Cyclical forces are then exerted on the stator and its supporting structure as the rotor rotates, giving rise to vibrations and noise. Excessive vibrations in combination with a high static load may also lead to material fatigue and damages of the supporting structure parts. In particular, such vibration and load problems are serious and difficult to combat in the case of very large and heavy machines, such as electric generators in power plants.
Moreover, sudden shock loads may occur in electric generators due to short circuit conditions or synchronising faults. For example, an electric generator may experience instant tangential torque loads as high as twenty times, or more, the normal operating torque during a synchronising fault, which creates a shock pulse in the machine. In the worst case, the supporting structure may be damaged and accidents may happen.
Typically, a stator core of an electric generator is built up from metal sheets formed as segments, distributed circumferentially around the rotor, and being further axially packed in plural layers. Each sheet segment in one layer overlaps two sheet segments in an adjacent layer, and the sheet package is axially clamped together as a rigid ring-shaped or tube-shaped structure by means of axially extending clamping bolts or the like.
The stator core is thus subjected to radial forces and counter-forces due to the magnetic field as the rotor rotates, which result in an oscillating elastic deformation of the core mainly in a radial direction. These radial oscillations tend to be transmitted over the supporting structure to the foundation upon which the generator rests, imposing vibrations thereto, which are undesirable for reasons mentioned above.
Various solutions have been proposed previously to isolate such vibrations from the foundation. Vertical leaf springs are commonly used for suspending a stator core, absorbing stator movements substantially in a horizontal plane, by elastic deformation of the leaf springs.
U.S. Pat. No. 6,091,177 discloses a suspension structure, where the stator core frame 11 of an electric generator is connected to a tapered spring bar 12 at axially dispersed attachment points 13. The spring bar 12 is connected to the upper part of a vertically extending spring plate 15 at axially dispersed connection points 16, interposed between said attachment points 13. The lower part of the spring plate 15 is finally connected to a foundation 17.
This suspension structure is resilient in a horizontal plane by means of the spring plate 15, and also in a vertical plane by means of the tapered spring bar 12. The suspension structure further includes a horizontally extending stabilisation bar 19 to provide necessary horizontal support. The spring plate 15 provides vertical support for the stator's dead weight and isolates horizontal vibrations of the stator from the foundation, whereas the tapered spring bar 12 is intended to isolate vertical vibrations.
However, this arrangement requires that the suspension parts are carefully designed in order to “tune” a resonant torsional frequency of the system, to minimise vibrations transmitted to the foundation. In general, suspension structures similar to the one described above, i.e., using resilient spring elements for absorbing vibrations, must be carefully designed and optimised if both the vibration isolation and the necessary static load support are to be achieved. A relatively stiff structure provides a solid static load support, but is less successful in isolating vibrations, and vice versa. Moreover, the entire deflection movement and vibration energy of a stator core is taken up by deformation of the spring elements, which puts high demands on the spring elements to withstand wear and fatigue.
It is highly desirable to isolate a foundation from vibrations in a stator core of an electric generator, in order to avoid noise and fatigue. It is also desirable to reduce dynamic loads from stator vibrations imposed on the support structure parts. It is also desirable to reduce the effects of shock forces on the foundation and the support structure. Further, it is desirable to provide a simple suspension structure, which does not require a high design accuracy and complexity, yet being reliable in withstanding static, dynamic and shock loads, also minimising noise and fatigue.